Power switching assembly having a robust gate connection

ABSTRACT

A structurally robust power switching assembly, that has a power transistor, comprising a thin and delicate layer of metal oxide, and a major surface of the layer of metal oxide being substantially coincident with a major surface of the power transistor, the major surface of the power transistor defining both an emitter and a gate. Also, dielectric material is placed over a portion of the emitter, so that it abuts the gate and a highly conductive pillar is constructed out of a relatively soft material, supported by the gate and the dielectric material, so that it has a larger area than would be possible if it was supported only by the gate.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application serial number 11/985,344 filed Nov. 14, 2007, now U.S. Pat. No. 8,089,150 which claimed priority from U.S. Provisional Application No. 60/858,801, filed Nov. 14, 2006.

BACKGROUND OF THE INVENTION

Power switching assemblies have many uses, including converting AC current to DC current and converting DC current to AC current. DC current may be converted to AC current for the purpose of driving an AC motor or as part of a process for converting DC current at a first frequency and/or voltage to DC current at a second frequency and/or voltage. These conversions and mechanisms for accomplishing these conversions are frequently necessary in the electrical engineering world. As fossil fuels become rarer, it is likely that the need to perform these conversions will become even more common, as electrical energy is gleaned from nature-dictated, rarely tapped sources, such as sun, wind and ocean movement, and processed into forms that electrical appliances have been built to accept.

A few fundamental problems face designers of power switching assemblies. Whereas the transistors, typically metal oxide semiconductor field effect transistors (MOSFETS) or insulated gate bipolar transistors (IGBTS) are made largely of silicon oxide crystal, which has a coefficient of thermal expansion (COTE) of about 2.3.

Although it is advantageous to use a high thermal conductivity material to draw the heat out of the transistors, only diamond has both high thermal conductivity and a low COTE. Accordingly, most high thermal conductivity materials used create a COTE mismatch at some point in the system.

In addition, both MOSFETs and IGBTs have a very thin (@100 nm), very delicate layer of metal oxide on a major surface of the silicon oxide substrate. Great care must be taken in electrically connecting this layer, to avoid a connection that would physically damage the metal oxide layer during manufacturing or thermal cycling.

In a separate development, due to an ability to withstand higher temperatures than other popular semiconductors used for power switches, silicon carbide metal semiconductor field effect transistors (MESFETS) are gaining popularity in the power semiconductor field.

SUMMARY

The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements.

In a first separate aspect, the present invention may take the form of a structurally robust power switching assembly, comprising a first rigid structural unit that defines a first unit major surface that is patterned to define a plurality of mutually electrically isolated, electrically conductive paths. Also, a similar, second rigid structural unit is spaced apart from the first unit major surface. Finally, a transistor is interposed between and electrically connected to the first unit major surface and the second unit major surface.

In a second separate aspect, the present invention may take the form of a structurally robust power switching assembly, comprises a power transistor that includes a thin and delicate layer of metal oxide. A major surface of the layer of metal oxide is substantially coincident with a major surface of the power transistor. Also, the major surface of the power transistor defines an emitter. In addition, a set of highly conductive pillars constructed out of a relatively soft material, supported by the emitter. A rigid structure is thermally mismatched to the major surface of the transistor and defines an electrically conductive pathway that is rigidly and electrically connected to the set of pillars. Finally, the highly conductive pillars are adapted to mitigate stress on the layer of metal oxide due to the thermal expansion mismatch between the rigid structure and the major surface of the transistor.

In a third separate aspect, the present invention may take the form of a structurally robust power switching assembly that includes a power transistor, comprising a thin and delicate layer of metal oxide, and a major surface of the layer of metal oxide being substantially coincident with a major surface of the power transistor, the major surface of the power transistor defining both an emitter and a gate. Dielectric material is placed over a portion of the emitter, and abutting the gate and a highly conductive pillar constructed out of a relatively soft material is supported by the gate and the dielectric material, so that it has a larger area than would be possible if it was supported only by the gate.

In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following detailed descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.

FIG. 1 is a circuit diagram, showing the circuit physically implemented in FIGS. 3, 10 and 11.

FIG. 2 is a top-perspective view of a work piece that represents a stage in the manufacturing of an electrical switching assembly.

FIG. 3 is a top-perspective view of the work piece of FIG. 1 at a later stage in the manufacturing process.

FIG. 4A is a perspective view of the gate and emitter side of an IGBT having copper pillars to ease the manufacturing process and conduct heat.

FIG. 4B is a perspective view of the collector side of the IGBT of FIG. 3.

FIG. 5 is a top perspective view of the work piece of FIG. 3, showing electrical connectors that have been attached.

FIG. 6 shows the work piece of FIG. 5, further having a housing.

FIG. 7 is a work piece used in the construction of an alternative embodiment of a switching circuit.

FIG. 8 is an additional work piece used in the construction of the same alternative embodiment.

FIG. 9A shows a further step in the production of the alternative embodiment.

FIG. 9B shows a step that immediately follows the step of FIG. 9A in the construction of the alternative embodiment.

FIG. 10 shows a completed switching assembly.

FIG. 11 shows the switching assembly of FIG. 10, in a housing and with electrical connectors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In a first preferred embodiment the present invention takes the form of a switching assembly 10 according to the schematic diagram of FIG. 1. A first pair insulated gate bipolar transistor (IGBTs) U_(H), placed in parallel, each include an emitter that is tied to a high voltage node N_(H), for example 48 VDC. The collector of the first IGBT pair U_(H) is the output node N_(O), on which a square wave voltage typically would be produced by circuit 10, in operation. A second IGBT pair U_(L), each has a collector that is connected to the output node N_(O) and an emitter that is connected to a low voltage (e.g. −48 VDC) or ground node N_(L). In operation, the gates of IGBTs U_(H) and U_(L) are toggled on and off, to alternately permit current to flow from the high voltage node N_(H) to the output node N_(O) and from output node N_(O) to the low voltage node N_(L).

As both IGBT pairs U_(H) and U_(L) cannot be on simultaneously there is typically a brief moment during switching when both IGBT pairs are off. Because the current through output node N_(O) cannot change instantaneously, a pair of diodes D_(H) bleed the current flowing from N_(O) up to the high voltage node N_(H), immediately after IGBTS U_(L) have turned off and from low voltage node N_(L) to output node N_(O) immediately after IGBTS U_(H) have turned off.

Two challenges that designers face when developing a physical implementation of this circuit are minimizing losses due to circuit inductance and accommodating thermal mismatch between the semiconductor material of IGBT pairs U_(H) and U_(L) and the thermally conductive material, such as copper, used to sink heat away from IGBT pairs U_(H) and U_(L), to prevent them from overheating. Moreover, the IGBTs U_(H) and U_(L) are planar and have a delicate layer of metal oxide on a major surface, which defines the emitter and the gate.

Referring to FIGS. 2-4, the switching circuit 10 may take physical form in an assembly 30. In order to accommodate the thermal mismatch and to reduce the risk of damaging the metal oxide layer during manufacturing, a set of copper pillars 40 and 42 (FIG. 4A) are grown on the emitter and gate, respectively, of IGBTs U_(H) and U_(L). The pillars 40 on the emitters of IGBTs U_(H) are electrically and physically connected, typically by flow soldering, to an output node electrically conductive pathway C_(NO) defined on the surface of a direct bond metal (DBM) substrate DBMU. The single pillar 42 on the gate is electrically and physically connected to a high-transistor gate electrically-conductive pathway C_(GH) on the same DBM substrate DBMU. As is well known in the art, the collector surface of an IGBT is far more resilient then the emitter and gate surface. Accordingly the collector of U_(H) is electrically and physically attached to a high node metal (typically copper) strap S_(NH), which is attached to a high-node electrically conductive pathway C_(NH) on DBM unit DBMU. The connection of strap S_(NH) may be accomplished also with flow soldering, but with a solder having a lower melting point and at a lower temperature than that previously used for the DBM connections, to avoid disturbing the DBM connections.

As indicated in FIG. 1, the collector of each low node IGBT U_(L) is connected to the emitter of the corresponding high node IGBT U_(L). Physically, this is accomplished by an additional copper strap S_(NO) (FIG. 3) that is connected to the collector of IGBT U_(L) and to the output-node pathway C_(NO). Much in the same manner as the emitter of IGBTs U_(H) (and contemporaneously) the emitter of IGBT U_(L) is flow soldered to a low-node conductive pathway C_(NL). Referring to FIG. 5, conductive pathways C_(NH), C_(NO) and C_(NL) form robust platforms for power connectors 56 which are flow soldered on. Returning to FIG. 3, Copper straps S_(NH) and S_(NO) each terminate in a copper pathway C_(NH) 2 and C_(NO) 2, respectively, which have no electrical function but help to draw heat away from the IGBTs U_(H) and U_(L). Finally, a housing 58 (FIG. 6) is introduced containing captured nuts 60, to facilitate connection to a set of busses. Housing 58 can also serve as a coolant conduit.

FIGS. 7-11 show a method of constructing an alternative preferred embodiment 130 (FIGS. 10 and 11). Referring to FIG. 7, a first DBM unit DBU_(HL), which includes both high voltage node N_(H) and low voltage node N_(L), in the form of two metalized regions, separated by a break in the metal (typically copper) and each having a tab T_(H) and T_(L), respectively, for connection to high and low voltage busses, respectively.

Each of two high-node IGBTS U_(H) is soldered to node N_(H) by way of its collector, which is quite hardy. The exposed, active surfaces of IGBTS U_(H) define a gate and emitter, both of which are covered with copper pillars 140 (FIGS. 9A and 9B). The two diodes D_(H) are also soldered to the same node and perform the same function as in the above described embodiment.

Referring to FIG. 8, a second DBM unit DBU_(O) has a parallel construction to DBU_(HL), except for that it has just a single node N_(O) and a single tab T_(O), for connection to an output buss. A pair of IGBTs U_(L) is soldered to node N_(O) by way of the IGBT collectors, as are diodes D_(L). Similar to IGBTs U_(H) the gate and emitter of U_(HL) are covered with copper pillars.

Referring to FIGS. 9A and 9B, in the manufacturing process, unit DBU_(HL) and DBU_(O) are faced toward each other so that the bump covered 140 emitters of IGBTs U_(H) both face node N_(O) and the similarly covered emitters of IGBTS U_(L) each face node N_(L). Then units DBU_(HL) and DBU_(O) are flow soldered together, forming a circuit 10 (FIG. 1), in a finished physical switching assembly 130 (FIG. 10). Skilled persons will recognize that the gates of IGBTS U_(H) and U_(L) may be connected by a copper wire, previously attached. Referring to FIG. 11 buss terminals 156 and a fluid-containing housing 158 complete the assembly 130.

Skilled persons will readily appreciate that assembly 130 offers many advantages over prior art switching systems. The copper node regions N_(H), N_(L), and N_(O) have a large surface area and volume, providing a robust path for current flow. Moreover, the emitters of IGBTs U_(N) are brought about as close as physically possible to the collectors of IGBTs U_(L), minimizing the current flow distance. Further, heat is free to exit IGBTs U_(H) and U_(L) through both DBU_(HL) and DBU_(O).

In another advantage, the exterior of the finished assembly 130 is electrically passive. Accordingly if the inside is sealed against fluid flow by introducing a sealing agent around the margins of assembly 130, it may be cooled with a conductive fluid such as water, or a water base solution. In addition, thermally conductive protrusions may be added to the exterior sides of assembly 116 aiding the exhausting of heat.

One design tool which may be used in the construction of either assembly 30 or assembly 130 addresses the problem of the small area presented by the gate of an IGBT. If a copper strap is to be joined to a copper pillar 42 on the IGBT gate, it may prove quite difficult to align it with a pillar the size of an IGBT gate, prior to soldering. Accordingly, an insulating sheet material may be deposited onto a portion of the IGBT emitter immediately adjacent to the gate, and the copper pillar for the gate may be grown, in part, on this insulating sheet so that it can have a larger surface area than would be otherwise possible. This facilitates the alignment operation in attaching the gate to a copper strap. For the case in which the gate is to be attached to a DBC unit, the larger surface area of the copper pillar facilitates a robust attachment.

While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. For example, although power transistors U_(H) and U_(L) are IGBTs in the above described embodiment, in an additional embodiment they are silicon carbide MESFETs and D_(H) and D_(L) are silicon carbide Schottky diodes. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope. 

The invention claimed is:
 1. A structurally robust power switching assembly, comprising: (a) a power transistor, comprising a thin and delicate layer of metal oxide, and a major surface of said layer of metal oxide being substantially coincident with a major surface of said power transistor, said major surface of said power transistor defining both an emitter and a gate; (b) dielectric material placed over a portion of said emitter, and abutting said gate; and (c) a highly conductive pillar constructed out of a relatively soft material, supported by said gate and said dielectric material, so that it has a larger area than would be possible if it was supported only by said gate.
 2. The switching assembly of claim 1 wherein a copper strap is joined to said highly conductive pillar.
 3. The switching assembly of claim 1 wherein a direct bond metal ceramic unit is contacted by said highly conductive pillar. 